1. Field of the Invention
The present invention relates to an iron-based alloy for use as a material for high-pressure components with increased working temperature. In particular, it relates to A heat-treatable steel for components such as tube heat exchangers in polyethylene high-pressure installations. This steel comprises the following main alloying elements in % by weight:
Carbon (C)0.22 to 0.29Chromium (Cr)1.1 to 1.5Molybdenum (Mo)0.3 to 0.6Nickel (Ni)3.3 to 3.7optionally, Vanadium (V)0.05 to 0.15the balance being iron (Fe).The still further comprises sulfide-forming and oxide-forming elements as well as accompanying and impurity elements. Furthermore, the invention relates to a component with increased working temperature, in particular a tube heat exchanger for polyethylene high-pressure installations made of an above-mentioned iron-based alloy.
2. Discussion of Background Information
Iron-based alloys according to DIN material no. 1.6604 or material no. 1.6580 or material no. 1.6586 and material no. 1.6926 or material no. 1.6944 and material no. 1.6952 are mostly used as materials for components that have to withstand high mechanical stresses at elevated temperatures, e.g., at 300° to 400° C., such as tube heat exchangers of chemical installations with an internal pressure of about 3,000 bar and higher. To establish the desired material strength, the parts are austenitized and hardened from the austenitizing temperature at a high cooling rate or quenched and then tempered, a stress-relieving treatment at temperatures up to the tempering temperature often following this heat treatment of the material.
A heat treatment by hardening and tempering for increasing the tensile strength of the material has a considerable impact also on other mechanical properties of the material at room temperature and at elevated working temperatures. An increase in the tensile strength above a value of about 1000 N/mm2 to about 1100 N/mm2 and higher disproportionally increases the 0.2% yield point of the iron-based material, whereby a ratio that is characteristic of the safety of the operation of high-pressure installations, i.e., the ratio of the 0.2% yield point (Rp0.2) and the tensile strength (Rm) is adversely influenced. In other words, the yield point approaches the tensile strength, with the elongation at break and the notch impact strength of the material being considerably reduced and the tear fracture toughness being substantially lowered.
For reasons of operating safety of high-pressure components, in particular that of installations of the chemical industry, the above-mentioned materials are heat-treated only up to the strength at which the associated elongation and toughness properties of the material are deemed to be sufficient or meet the regulations. A disadvantage in terms of installation engineering is that a great wall thickness of the high-pressure components is thus necessary, and there may also be an influence on reaction kinetics of the chemical materials and a low cost-effectiveness of the reactor or the installation. If, e.g., high-pressure heat exchangers are designed to establish sufficiently high elongation and toughness values of the material with the necessary strength of the same, the wall thickness has to be given large dimensions according to the stress, which is associated with a low specific heat transmission, which necessitates large thick-walled reactors.
A difficulty associated with thick-walled tubes is meeting the so-called “leak prior to fracture” criterion that always has to be met in high-pressure technology for safety reasons. In other words, if in the operation of a reactor a crack grows in the tube wall, this crack first has to reach the outer surface (=leak), before an unstable fracture occurs. The critical fracture toughnesses, such as K1c or J1c or the critical crack length ac are characteristic values of an unstable fracture. These material-specific characteristic values depend primarily on the toughness of the material.
It is desirable to overcome the above deficiencies. In particular, it would be advantageous to provide an iron-based alloy of the type mentioned at the outset for use in high-pressure components with increased strength at high elongation and toughness values of the material.
It would also be advantageous to provide a component, in particular, a tube heat exchanger for polyethylene high-pressure installations, with improved performance characteristics and/or similar safety criteria, which component is made of an above-mentioned iron-based material with high strength and at the same time favorable elongation and toughness values.